Circuits for power factor correction (hereinafter PFC) nowadays play an important role in current and/or voltage converters. For small to middle power ranges, e.g. 25 W to 150 W, power factor correctors working with currents of triangular shape are commonly used, wherein in each switching cycle a magnetic energy stored in an inductor is completely released and a new switching cycle is initiated immediately after the magnetic energy is completely released which can be detected via an auxiliary coil of the inductor. The advantage of power factor correctors relying on triangular currents is a relatively slow rate of current change in a rectifier which makes it possible to use inexpensive rectifying diodes. However, the drawbacks involved are the variable operation frequency and a high ripple of the input current. The latter issue can be resolved by using a power factor corrector working with currents of trapezoidal shape which is mostly used for power ranges between 150 W and 500 W. The operating frequency is usually fixed and is set by a clock generator. A major disadvantage of the use of a power factor corrector working with currents of trapezoidal shape is the need to use a relatively expensive fast rectifying diode.
To achieve even higher power ranges, multiphase power factor correctors working with currents of trapezoidal shape are used, where a multiphase clock generator provides the separate power factor correction stages with phase shifted clock signals which are phase shifted by 360°/N with respect to one another, wherein N denotes the number of power factor correction stages which is also equivalent to the number of different phases.
Over the past, multiphase power factor correctors working with currents of triangular shape have become increasingly attractive and partly replace single phase power factor correctors working with currents of trapezoidal shape in some applications in the range starting from 150 W. The multiphase power factor correctors working with currents of triangular shape offer several advantages over single phase power factor correctors working with currents of trapezoidal shape. The switching losses are considerably lower, which for example allows for a higher operating frequency and in turn may result in smaller magnetic components or a higher efficiency. As the magnetic components may have a decreased height, the overall circuitry can be constructed in a more compact fashion and hence their integration into flat screens, for example, may be simplified.
In case of an interleaved PFC circuit which includes more than one PFC stage working with currents of triangular shape, the synchronisation of PFC stages with respect to one another is a non-trivial task to be solved, since each PFC stage is a self-oscillating system which operates independent of a cycle generator.
In a conventional method to synchronise multiphase power factor correctors working with currents of triangular shape, two power factor correctors working with currents of triangular shape are operated independent of one another. Signals for setting the duration of the on-time of each power factor corrector stage are modulated depending on an averaged phase shift between the signals, such that by means of a phase control the power factor corrector stages reach a state of running out of phase. In another approach, a signal of a zero crossing detector of one of the two power factor corrector stages is delayed depending on an averaged phase difference between the two power factor corrector stages.